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Tuesday, March 15, 2011

Spent Nuke Fuel Pool May Be Boiling, Further Radiation Leak Feared

Kyodo News, Tokyo, Mar 15, 2011
A nuclear crisis at the quake-hit Fukushima No. 1 [Daiichi] nuclear plant deepened Tuesday as fresh explosions occurred at the site and its operator said water in a pool storing spent nuclear fuel rods may be boiling, an ominous sign for the release of high-level radioactive materials from the fuel.

Tokyo Electric Power Co. said water levels in the pool storing the spent fuel rods at its Fukushima No. 1 nuclear plant's No. 4 reactor may have dropped, exposing the rods. The firm said it has not yet confirmed the current water levels or started operations to pour water into the facility.

Unless the spent fuel rods are cooled down, they could be damaged and emit radioactive substances. The government's Nuclear and Industrial Safety Agency urged TEPCO to inject water into the pool soon to prevent heating of the fuel rods.

At 6:14 a.m., a blast occurred at the No. 4 reactor and created two square holes sized about 8 meters by 8 meters in the walls of the building that houses the reactor. At 9:38 a.m., a fire broke out there and smoke billowed from those holes.

The utility said it cannot deny the possibility that the early morning explosion was caused by hydrogen that was generated by a chemical reaction involving the exposed spent nuclear fuel and vapor.

Chief Cabinet Secretary Yukio Edano said at a press conference, ''We believe very-high-level radioactive substance has not been emitted continuously from the No. 4 reactor,'' citing radiation monitoring data at the plant.

The nuclear agency said water temperature in the pool stood at 84 degrees C as of 4 a.m. Monday, higher than the normal level of 40 to 50 degrees. Usually, the upper tip of the fuel rods is at a depth of 10 meters from the surface of the pool, it said.

Agency officials said the fuel rods will not reach criticality again as they have been stored in racks containing boron to prevent the phenomenon.

Edano said water temperature in the pools at the No. 5 and No. 6 reactors in the Fukushima plant has been rising as well.

The three reactors were not in service when a magnitude 9.0 earthquake jolted Fukushima Prefecture and other areas in northeastern Japan on Friday.

The agency said among the three, the situation is the severest for the No. 4 reactor because all the fuel rods are stored in the pool due to the change of the reactor's shroud. At the No. 5 and No. 6 reactors, up to one-third of the rods are kept in the pools.

The new development followed a critical situation at the No. 2 reactor at the Fukushima plant earlier in the day, in which part of the reactor's container vessel was damaged following an apparent hydrogen explosion at 6:10 a.m.

TEPCO said the problem could develop into a critical ''meltdown'' situation, in which fuel rods melt and are destroyed, emitting massive radioactive materials in the air.

Prime Minister Naoto Kan urged people living between 20 and 30 kilometers of the plant to stay indoors, after radiation equivalent to 400 times the level to which people can safely be exposed in one year was detected near the No. 3 reactor in the plant.

Residents within a 20-km [12-mile] radius have already been ordered to vacate the area following Saturday's hydrogen blast at the plant's No. 1 reactor.

''The danger of further radiation leaks (from the plant) is increasing.'' Kan warned the public at a press conference, while asking people to ''act calmly.''

Edano said the high radiation level detected at 10:22 a.m. after the explosions at the No. 2 and No. 4 reactors ''would certainly have negative effects on the human body.''

The top government spokesman said later in the day the massive radiation amount may have been recorded around the debris of the building that housed the No. 3 reactor. The building was blown away by the hydrogen explosion Monday.

TEPCO has been continuing operations to pour seawater into the troubled No. 1, 2 and 3 reactors to prevent overheating and further damage to their containers. But despite the water injection, fuel rods in the three reactors remain partially exposed.

The cores of those three reactors at the plant are believed to have partially melted following the devastating quake.

The country's biggest recorded quake, which is also one of the largest in global history, caused the three reactors, which were all operating at the time, to automatically shut down. [Full story]